1. Field of the Invention
The present invention generally relates to coatings of the type used to protect components exposed to high temperature environments, such as bond coats and environmental coatings for gas turbine engine components. More particularly, this invention is directed to a ceramic-containing beta-phase (Î2) NiAl (beta-NiAl) overlay coating for use as a bond coat or environmental coating.
2. Description of the Related Art
Components within the turbine, combustor and augmentor sections of gas turbine engines are susceptible to oxidation and hot corrosion attack, in addition to high temperatures that can decrease their mechanical properties. Consequently, these components are often protected by an environmental coating alone or in combination with an outer thermal barrier coating (TBC), which in the latter case is termed a TBC system. Ceramic materials such as zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO) or other oxides, are widely used as TBC materials.
Various metallic coating systems have been used as environmental coatings for gas turbine engine components, the most widely used being diffusion coatings such as diffusion aluminides and platinum aluminides (PtAl), and MCrAlX overlay coatings (where M is iron, cobalt and/or nickel, and X is an active element such as yttrium or another rare earth or reactive element). Used in combination with TBC, a diffusion aluminide or MCrAlX overlay coating serves as a bond coat to adhere the TBC to the underlying substrate. The aluminum content of these bond coat materials provides for the slow growth of a strong adherent continuous aluminum oxide layer (alumina scale) that protects the bond coat and underlying substrate from oxid.
Diffusion and MCrAlX bond coats containing ceramic particles have been evaluated. For example, commonly-assigned U.S. Pat. Nos. 6,168,874 to Gupta et al. and 6,485,845 to Wustman et al. incorporate oxide particles in diffusion aluminide coatings to slow oxide scale growth, thereby increasing the spallation resistance of a TBC. Furthermore, commonly-assigned U.S. Pat. No. 4,101,713 to Hirsch et al. discloses that an oxide dispersion-strengthened MCrAlY coating exhibits improved mechanical integrity. Others, such as U.S. Pat. No. 4,447,503 to Dardi et al., disclose that oxide particles in an MCrAlY coating promote pinning protective oxide scales, while still others, such as U.S. Pat. No. 4,451,496 to Gedwill et al., U.S. Pat. No. 6,306,515 to Goedjen et al. and U.S. Pat. No. 6,376,015 to Rickerby, disclose the use of oxide particles in MCrAlY as an inhibitor to interdiffusion between an underlying substrate and an environmental coating deposited on the MCrAlY coating. The incorporation of oxide particles in an MCrAlY for the purpose of modifying its coefficient of thermal expansion has also been suggested, e.g., U.S. Pat. No. 6,093,454 to Brindley et al., EP 0 799 904 to Movchan et al., and EP 0 340 791 to Kojima et al. Finally, the incorporation of other types of ceramic particles in bond coat materials has been suggested, as reported in U.S. Pat. No. 6,291,014 to Warnes et al. (suicides and carbides for high temperature oxidation resistance).
More recently, overlay coatings of predominantly beta-nickel aluminide intermetallic have been proposed as environmental and bond coat materials. The NiAl beta phase exists for nickel-aluminum compositions of about 30 to about 60 atomic percent aluminum, the balance of the nickel-aluminum composition being nickel. Notable examples of beta-NiAl coating materials include commonly-assigned U.S. Pat. No. 5,975,852 to Nagaraj et al., which discloses a NiAl overlay bond coat optionally containing one or more reactive elements, such as yttrium, cerium, zirconium or hafnium, and commonly-assigned U.S. Pat. No. 6,291,084 to Darolia et al., which discloses a NiAl overlay coating material containing chromium and zirconium. Commonly-assigned U.S. Pat. Nos. 6,153,313 and 6,255,001 to Rigney et al. and Darolia, respectively, and commonly-assigned U.S. Patent No. 6,620,524 to Pfaendtner et al. also disclose beta-phase NiAl bond coat and environmental coating materials. The alloying additions to these beta-NiAl coating materials have been shown to improve the adhesion of a ceramic TBC layer, thereby inhibiting spallation of the TBC and increasing the service life of the TBC system.
NiAlCrZr overlay coatings described in the above-noted commonly-assigned patents derive their performance benefits from optimum combinations of aluminum and the reactive elements, chromium and zirconium. At certain levels, zirconium promotes an adherent slow growing (low values of the parabolic scale growth parameter, kp) alumina scale, which helps to extend the TBC spallation life and improve oxidation performance. While oxidation performance suffers if the zirconium level is too low (e.g., below 0.05 atomic percent), higher levels of zirconium result in Zr-rich intermetallic precipitates that can increase internal oxidation. In spite of this internal oxidation phenomenon, levels of zirconium above 0.2 atomic percent (about 0.4 weight percent) have shown to significantly improve TBC spallation resistance as a result of the potent strengthening effect of zirconium additions to beta-NiAl alloys. Strengthening in beta-NiAl by zirconium additions has been attributed to two mechanisms: solid solution strengthening, and the formation of zirconium-containing intermetallic precipitates, the most common being a Heusler phase (Ni2AlZr) that results in further ordering of the NiAl structure. The increased strength of beta-phase NiAl-based bond coats has been shown to contribute to better TBC lives.
However, and as mentioned above, the higher zirconium levels required to optimize TBC spallation resistance also promote internal oxidation (oxidation of Zr-intermetallic precipitates), which degrades the overall oxidation resistance of the bond coat by effectively increasing the parabolic scale growth parameter, kp. While it is critical that a bond coat provide TBC spallation resistance, bond coats must also exhibit oxidation resistance in the event of TBC spallation. Therefore, further improvements are needed in beta-phase NiAl-based overlay coatings that can result in both improved oxidation resistance and, if used as a bond coat, improved spallation resistance.